You can't run an airline that is free to exclude any passenger at will. TSA pre-check is based on the fallacious inference that rich frequent flyers and terrorists are a distinct set. Saudi royal-funded operatives may well be more likely to be frequent flyers than the general population.

It's more that if you didn't flag them the first N times you're rather unlikely to flag them in the future. The screening isn't perfect but screening these people more won't help.